Unlike man made materials such as concrete, steel, etc, natural materials such as rocks and soils are subject to natural (virgin) stresses called in-situ stresses. Knowledge of the in-situ stress field is very important in many problems dealing with rocks in civil, mining and petroleum engineering as well as in geology and geophysics. The last 30 years have seen a major advance in our knowledge of in-situ stresses. A large body of data on the state of stress in the near surface of the Earth's crust (upper 4-5 km of the crust) is now available. New theories have been proposed regarding their origin and various techniques have been developed to measure in-situ stresses.